Electrochromism is a term used to describe a reversible change of color exhibited by some materials when placed in an electric field. In general, electrochromic materials are substances whose optical properties can be reversibly altered through application of an electric current. Some materials can exhibit a mirror-like state when no electric field is present, and change to a transparent state when an electric field is applied, and vice versa. Devices capable of switching between mirror-like and transparent states find many applications in architectural and transportation energy conservation, lighting and displays, aerospace insulation control, and optical communications systems. Such devices, termed “switchable mirrors”, based on rare earth hydrides were discovered by Huiberts et al., Nature 380, 231 (1996), who observed a reversible metal-to-insulator transition when a thin film (150 to 500 nm) of yttrium or lanthanum coated with a thin layer of palladium was exposed to hydrogen gas. The transition accompanies conversion of a metallic dihydride phase to a semiconducting trihydride. Rare earth-magnesium alloy films were subsequently found to be superior to the pure lanthanides in maximum transparency and mirror-state reflectivity, see Van der Sluis et al., Appl. Phys. Lett. 70, 3356 (1997).
Variable reflectance coatings (switchable mirrors) have significant advantages over traditional absorbing devices for radiant energy control in a variety of architectural and aerospace applications due to their large dynamic ranges in both transmission and reflection in the visible and near infrared regimes. Although electrochromic and gasochromic metal hydride films have been the primary focus of recent developments in this field, the instant invention contemplates electrochromic devices using materials other than the known metal hydrides.